This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Superabsorbent Poly(isoprenecarboxylate) Hydrogels from Glucose Grant W. Fahnhorst and Thomas R. Hoye* Department of Chemistry, University of Minnesota, 207 Pleasant St. SE,Minneapolis, Minnesota 55455, United States
ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by 95.85.68.214 on 03/25/19. For personal use only.
S Supporting Information *
ABSTRACT: Isoprenecarboxylic acid (ICA-H), available from glucose via one of its major metabolites mevalonate, has been converted to cross-linked networks by radical polymerization. Monomer feeds comprising various ratios of ICA-H and its sodium salt (ICA-Na) were used to give hydrogels that show attractive performance in comparison with (nonbioderived) poly(acrylate) hydrogels. In particular, these new materials show increasing levels of water uptake (i.e., swelling ratio) across the entire range of ionization (10−90 %Na). This behavior is attributed to the larger distance between carboxylate moieties in the hydrogels, a feature that reduces the average amount of charge repulsion between proximal sodium carboxylate ion pairs (counterion condensation). KEYWORDS: Hydrogels, Bioderived, Superabsorbent
■
INTRODUCTION Hydrogelscross-linked polymer networks that swell in the presence of waterare widely used in everyday applications. Their unique ability to retain water have made them suitable for applications ranging from personal care commodity and specialty products such as diapers, shampoos, lotions, and cosmetics to more sophisticated applications such as drugdelivery agents and contact lenses. Most commonly, hydrogels are derived from water-soluble monomers of the acrylic acid family [e.g., (meth)acrylic acid, (meth)acrylamide, N-isopropylacrylamide, dimethylacrylamide, etc.]. Superabsorbent polymers (SAPs) are capable of retaining very large amounts of water [e.g., >100 times the mass of the dried polymer network (aka, dried gel)]. These typically comprise copolymers of ionizable carboxylic acids, portions of which are present as their corresponding alkali metal carboxylate salts (e.g., sodium acrylate with acrylic acid). The ionic nature of such copolymers provides the enthalpic driving force that renders them hydrophilic.1 With growing demand toward replacing petroleum-based polymers with sustainable alternatives, researchers have explored producing acrylic acid from renewable feedstocks like lactic acid,2−5 3-hydroxypropionic acid,6,7 and glycerol.8,9 Although technically feasible, at present, these are not deemed to be economically viable. Additionally, considerable research has been focused on incorporating other renewable monomers such as α-methylene-γ-butyrolactone,10 starch,11 alginates,12 cellulose,13−17 itaconic acid,18−20 and chitosan21,22 into hydrogels, but this nearly always involves incorporation of acrylate moieties for the polymerization chemistry. There remains a need for viable alternatives in which the content of the hydrogel polymer is largely bioderived. © XXXX American Chemical Society
Recently, we prepared isoprenecarboxylic acid (ICA-H, Figure 1) from anhydromevalonolactone, which can be
Figure 1. Previous work: conversion of isoprenecarboxylic acid (ICAH) to its corresponding esters ICA-R and their polymerization to poly(isoprenecarboxylic acid esters) [poly(ICA-R)].23
efficiently obtained from glucose via mevalonate.23 We proceeded to synthesize and radically polymerize the corresponding isoprenecarboxylate esters (ICA-R) to poly(isoprenecarboxylic acid esters) [poly(ICA-R)]. We observed that these behave similarly to poly(acrylates) in terms of entanglement molecular weights and glass transition temperatures. To expand on this technology, we envisioned that ICAH could be used as a new platform for the development of novel hydrogels and SAPs. This route is attractive because ICA-H has the potential to be produced at a price point where it perhaps could serve as a major component of a specialty hydrogel.24 This study was, therefore, undertaken to explore a potential practical application of this new polymer. Received: January 12, 2019 Revised: February 12, 2019
A
DOI: 10.1021/acssuschemeng.9b00218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
■
and ground to fine powders (see SI for details) having particle sizes ranging from the tens to low hundreds of μm (see representative SEM images in Figure S4). Overall, mass recovery of the final dried powder was typically in the range of 70%−90% based on monomer conversion. The IR spectra of the dried sodiated isoprenecarboxylic acid gels are shown in Figure 2c. With increasing ionization, the spectra show (i) a decrease in the intensity of the CO stretch corresponding to the acid repeat units (1688 cm−1) and (ii) an increase in that of two CO stretches at 1547 (asymmetric stretch) and 1395 (symmetric stretch) cm−1, indicative of the sodium carboxylate repeat units.28 Additionally, more water is incorporated within those dried gels that contain higher percentages of ICA-Na in the starting polymerization mixture as observed by the intensity of the broad OH stretch between ∼3100−3600 cm−1 [distinct from the underlying OH stretch from CO2H groups (ca. 2500− 3500 cm−1)]. This interpretation of water retention at higher %Na is supported by the observed early mass loss ( Na > K. In the presence of excess (5−8 fold) 0.17 M NaCl or KCl solution, the differences among the SRs as well as their overall magnitude are considerably compressed. We also briefly studied the effect of cross-link density within vs swelling, and the data are shown in Table 1. A reduction in SR is observed Gel(ICA-H/Na) as the number of cross-links increases within a given volume, a common phenomenon for many hydrogels.29
RESULTS AND DISCUSSION We first synthesized a series of sodiated poly(isoprenecarboxylates) [poly(ICA-H/Na)] as shown in Figure 2a. The relative number of carboxylates to carboxylic acids was
Figure 2. (a) Synthesis of linear poly(isoprenecarboxylic acid/ carboxylate) [poly(ICA-H/Na)]. (b) Synthesis of cross-linked hydrogels Gel(ICA-H/Na) from monomers ICA-H and ICA-Na and the cross-linking agent ICA-XL. (c) IR spectra of hydrogels containing varying amounts of sodium carboxylates.
controlled by varying the relative amounts of ICA-H and its sodium salt ICA-Na in the monomer feed by pretitrating the former with controlled amounts of NaOH. Use of 50:50 MeOH/H2O as the solvent allowed for maintenance of homogeneity at the reaction temperature (65 °C) throughout the entire polymerization. The ratio of the two monomers had little effect on the 1,2-mode vs 1,4-mode of addition in the radical polymerization. The ratio of these competing additions was assessed by 1H NMR analysis (D2O) of the resulting linear poly(ICA-H/Na); the value of 1.0:1.5 was the same as that observed in poly(ICA-R).23 However, the rate of the polymerization slowed as the percentage of sodium carboxylates in the feed was increased. This phenomenon is also reported for (meth)acrylic acid and sodium (meth)acrylate copolymerizations.25−27 We proceeded to use solution polymerization to synthesize cross-linked networks of sodiated poly(isoprenecarboxylic acid) [Gel(ICA-H/Na)] (Figure 2b). We used analogous conditions described for the linear poly(ICA-H/Na) preparation, now in the presence of 1 mol % of the cross-linking agent ICA-XL. Each sample was polymerized to the gel state, from which any soluble components leached with water and acetone. The resulting gels were dried (100 °C, vacuum) B
DOI: 10.1021/acssuschemeng.9b00218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
poly(acrylic acid)-based gels made by polymerization of acrylic acid/sodium acrylate feeds,30,31 we opted to synthesize our own Gel(AA-H/Na) for this comparative study by an analogous protocol to that used to prepare Gel(ICA-H/Na) (SI). The results for the swelling of Gel(AA-H/Na) are shown by the blue bars in Figures 3a and b. The most significant difference in the observed trends is that Gel(AA-H/Na) swelling reaches a maximum at 50% sodiation of its acid groups and then wanes at higher levels of ionization,32−34 whereas the Gel(ICA-H/Na) samples continue to increase their extent of swelling at increasingly higher amounts of sodiation. This behavior is parallel for the swelling data in either DI water (Figure 3a) or 0.17 M (1% w/v) NaCl solution (Figure 3b). This difference can be explained by a phenomenon known as counterion condensation.35−37 Because the average distance between any two carboxylate groups is smaller for Gel(AA-H/ Na) than for Gel(ICA-H/Na) (see red vs blue in Figure 4),
Figure 4. Average distance between carboxylates is larger for poly(ICA-Na) than for poly(AA-Na) (i.e, dICA > dAA). Representative substructure shown for the former (here for a pair of adjacent 1,2- and 1,4-ICA subunits) is one of several present in poly(ICA-Na).
the average “tightness” of the ion pairing of each Na+ with its carboxylate is greater in the former than in the latter in order to minimize repulsion between neighboring anionic carboxylates. This leads to a reduced driving force for water solvation of the sodium counterions present in Gel(AA-H/Na). From the alternative perspective, the carboxylate ions are less repulsive in Gel(ICA-H/Na), allowing its sodium counterions to take on higher levels of solvation. Thus, the greater average spacing between the carboxylates in Gel(ICA-H/Na) imparts a significant performance advantage to these novel hydrogels. Because each series of polymeric gels was made by using different ratios of RCO2H:RCO2Na monomer feeds, it is possible that differences in the morphologies of the resulting networks contribute to the differences in swelling behavior. This was briefly explored by taking a sample of the 50% sodiated material for each of the acrylate [Gel(AA-H/Na)] and isoprenecarboxylate [Gel(ICA-H/Na)] and titrating it to 90% sodiation by addition of aqueous NaOH. The SRs of the resulting hydrated gels, in each case, were within error, identical to those observed for the material made directly by starting with a 10:90 RCO2H:RCO2Na ratio of monomers [see diamonds in Figure 3a]. This suggests that any morphological differences associated with the method of gel synthesis is not contributing significantly to the differences in the trends for these two classes of hydrogels, further supporting the counterion condensation interpretation. We briefly explored the effect of pH on the swelling ratio38 of a single gel sample for each of Gel(ICA-H/Na) and Gel(AA-H/Na) (25 %Na in various buffered solutions at constant ionic strength; Table S5, Figure S10). The trends seen there mirror reasonably well those observed for the swelling studies portrayed in Figure 3.
Figure 3. Swelling ratio (SR = g of swollen gel/g of dried gel) vs percent sodiation in the starting monomer feed for both Gel(ICA-H/ Na) (red) and Gel(AA-H/Na) hydrogels (blue) prepared under identical conditions in (a) DI H2O and (b) 0.17 M (1% w/w) NaCl aqueous solution. Diamonds represent the SR in DI water for a sample of 90 %Na Gel(ICA-H/Na) or Gel(AA-H/Na) prepared by titrating a 50 %Na gel with additional NaOH.
Table 1. Effect of Alkali Metal Counterion (Met) on SR for 90% ionization of Li, Na, and K Gel(ICA-H/Met) and Effect of Cross-Link Density on SR for 90 %Na of Gel(ICA-H/Na) Alkali metal counterion vs swelling ratio for Gel(ICA-H/Met) M
DI H2O
0.17 M NaCl
0.17 M KCl
90 %Li 322 ± 12 56 ± 1 54 ± 2 90 %Na 258 ± 15 52 ± 1 48 ± 1 90 %K 224 ± 21 46 ± 0.3 44 ± 2 Cross-link density vs swelling ratio for Gel(ICA-H/Na) (90 %Na) Mol% ICA-XL 0.1 1.0 2.5 5.0
DI H2O
0.17 M NaCl
0.17 M KCl
No gel fraction detected 258 ± 15 185 ± 6 116 ± 3
No gel fraction detected 52 ± 1 48 ± 1 35 ± 2
No gel fraction detected 48 ± 1 49 ± 1 25 ± 2
Finally, we were also interested in benchmarking the swelling performance of these new bioresourced Gel(ICA-H/ Na) samples against that of samples of sodiated, cross-linked, poly(acrylic acid) gels [Gel(AA-H/Na)]. Because many factors are known to influence the precise nature of the C
DOI: 10.1021/acssuschemeng.9b00218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering Notes
The phenomenon of lack of counterion condensation across a full range of %ionization has been observed for poly(styrenesulfonic acid)39 and poly(2-acrylamido-2-methylpropanesulfonic acid),40 polymers in which the ionizable groups are also spatially separated to a greater extent than in poly(acrylic acid). However, we are not aware of this being observed previously for carboxylic acid-containing polymers or those that are bioresourcable. In addition, we have not seen examples of reports of continuous increase in SRs across the full spectrum of ionization.
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors appreciate guidance provided by Professor Ronald A. Siegel, University of Minnesota. Financial support for this research was provided by (i) the Center for Sustainable Polymers at the University of Minnesota, an NSF-supported Center for Chemical Innovation (CHE-1413862), and (ii) University of Minnesota Doctoral Dissertation Fellowship. Some of the NMR data were recorded on an instrument purchased with support of the NIH Shared Instrumentation Grant program (S10OD011952). SEM imaging was performed in the Characterization Facility at the University of Minnesota, which receives partial support from the NSF through the MRSEC program.
■
CONCLUSION Using the bioderived isoprenecarboxylic acid monomer ICAH, we have synthesized and studied the swelling properties of Gel(ICA-H/Na). A series of these cross-linked, polymeric hydrogels varying in the extent of ionization (i.e., %Na loading) of the carboxyl groups present in each backbone repeat unit was prepared. The swelling behavior of each gel was benchmarked against similar hydrogels that we made by the same polymerization protocol from acrylic acid/sodium acrylate monomer feeds. The novel Gel(ICA-H/Na) materials retain ever-increasing amounts of water across the entire range of %Na loading, a phenomenon that distinguishes them from and is advantageous to the behavior we observed for the analogous acrylate-based gels. The poly(acrylate)s [Gel(AAH/Na)] showed a maximum amount of swelling at 50 %Na (140 times in DI water), whereas the new Gel(ICA-H/Na) gels showed continuously increased uptake even at the highest (90 %Na) degree of ionization (260 times in DI water). We attribute this difference to a reduced amount of counterion condensation in the Gel(ICA-H/Na) samples because of the greater spatial separation of the carboxyl groups in the ICAbased gels, a potentially more broadly exploitable phenomenon. It is notable that (i) all of the carbon atoms in mevalonate, the organic raw material whose production from glucose by an engineered metabolic pathway24 has been performed on large scale, are present in the hydrogel; (ii) >99% of the entire carbon content of the gel is derived from mevalonate; and (iii) only straightforward chemistry (water is the sole byproduct) is needed for the three reactions that transform mevalonate into the network gel: an acid-catalyzed dehydration, a basemediated eliminative ring-opening, and a radical initiated polymerization.
■
■
(1) Thakur, S.; Thakur, V. K.; Arotiba, O. A. History, Classification, Properties and Application of Hydrogels: An Overview. In Hydrogels: Recent Advances, 1 ed.; Thakur, V. K.; Thakur, M. K., Eds.; Springer: Singapore, 2018; pp 29−50, DOI: 10.1007/978-981-10-6077-9_2.. (2) Aida, T. M.; Ikarashi, A.; Saito, Y.; Watanabe, M.; Smith, R. L., Jr.; Arai, K. Dehydration of lactic acid to acrylic acid in high temperature water at high pressures. J. Supercrit. Fluids 2009, 50, 257−264. (3) Yee, G. M.; Hillmyer, M. A.; Tonks, I. A. Bioderived acrylates from alkyl lactates via Pd-catalyzed hydroesterification. ACS Sustainable Chem. Eng. 2018, 6, 9579−9584. (4) Holmen, R. E. Acrylates by catalytic dehydration of lactic acid and lactates. U.S. Patent 2,859,240, November 4, 1958. (5) Sawicki, R. A. Catalyst for dehydration of lactic acid to acrylic acid. U.S. Patent 4,729,978, March 8, 1988. (6) Velasquez, J. E.; Collias, D. I.; Godlewski, J. E.; Wireko, F. C. Catalytic dehydration of hydroxypropionic acid and its derivatives. U.S. Patent 20170057900A1, March 2, 2017. (7) Li, C.; Zhu, Q.; Cui, Z.; Wang, B.; Fang, Y.; Tan, T. Highly efficient and selective production of acrylic acid from 3-hydroxypropionic acid over acidic heterogeneous catalysts. Chem. Eng. Sci. 2018, 183, 288. (8) Kim, M.; Lee, H. Highly selective production of acrylic acid from glycerol via two steps using Au/CeO2 catalysts. ACS Sustainable Chem. Eng. 2017, 5, 11371−11376. (9) Witsuthammakul, A.; Sooknoi, T. Direct conversion of glycerol to acrylic acid via integrated dehydration-oxidation bed system. Appl. Catal., A 2012, 413-414, 109−116. (10) Kollár, J.; Mrlík, M.; Moravčíková, D.; Kroneková, Z.; Liptaj, T.; Lacík, T.; Mosnácě k, J. Tulips: A renewable source of monomer for superabsorbent hydrogels. Macromolecules 2016, 49, 4047−4056. (11) Castel, D.; Ricard, A.; Audebert, R. Swelling of anionic and cationic starch-based superabsorbents in water and saline solution. J. Appl. Polym. Sci. 1990, 39, 11−29. (12) Chan, A. W.; Whitney, R. A.; Neufeld, R. J. Semisynthesis of a controlled stimuli-responsive alginate hydrogel. Biomacromolecules 2009, 10, 609−616. (13) Marcì, G.; Mele, G.; Palmisano, L.; Pulito, P.; Sannino, A. Environmentally sustainable production of cellulose-based superabsorbent hydrogels. Green Chem. 2006, 8, 439−444. (14) Sannino, A.; Demitri, C.; Madaghiele, M. Biodegradable cellulose-based hydrogels: Design and applications. Materials 2009, 2, 353−373. (15) Chang, C.; He, M.; Zhou, J.; Zhang, L. Swelling behaviors of pH- and salt-responsive cellulose-based hydrogels. Macromolecules 2011, 44, 1642−1648. (16) Dai, H.; Huang, H. Enhanced swelling and responsive properties of pineapple peel carboxymethyl cellulose-g-poly(acrylic
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00218.
■
REFERENCES
Experimental procedures for gel preparation, IR data for the dried gels, swelling data for saturated gels, and structural characterization, including copies of 1H and 13 C NMR spectra, for monomers ICA-H and ICA-XL. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Grant W. Fahnhorst: 0000-0002-2193-4246 Thomas R. Hoye: 0000-0001-9318-1477 D
DOI: 10.1021/acssuschemeng.9b00218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Letter
ACS Sustainable Chemistry & Engineering
(36) Stevens, M. J.; Kremer, K. The nature of flexible linear polyelectrolytes in salt free solution: A molecular dynamics study. J. Chem. Phys. 1995, 103, 1669−1690. (37) Rička, J.; Tanaka, T. Swelling of ionic gels: quantitative performance of the Donnan theory. Macromolecules 1984, 17, 2916− 2921. (38) Bao, Y.; Ma, J.; Li, N. Synthesis and swelling behaviors of sodium carboxymethyl cellulose-g-poly(AA-co-AM-co-AMPS)/MMT superabsorbent hydrogels. Carbohydr. Polym. 2011, 84, 76−82. (39) Kwak, J. C. T.; Hayes, R. C. Electrical conductivity of aqueous solutions of salts of polystyrenesulfonic acid with univalent and divalent counterions. J. Phys. Chem. 1975, 79, 265−269. (40) Gong, J. P.; Komatsu, N.; Nitta, T.; Osada, Y. Electrical conductance of polyelectrolyte gels. J. Phys. Chem. B 1997, 101, 740− 745.
acid-co-acrylamide) superabsorbent hydrogel by the introduction of carclazyte. J. Agric. Food Chem. 2017, 65, 565−574. (17) For a review on cellulose-based hydrogels see: Ma, J.; Li, X.; Bao, Y. Advances in cellulose-based superabsorbent hydrogels. RSC Adv. 2015, 5, 59745−59757. (18) Krušić, M. K.; Filipović, J. Copolymer hydrogels based on Nisopropylacrylamide and itaconic acid. Polymer 2006, 47, 148−155. (19) Karadaǧ, E.; Saraydin, D.; Güven, O. Radiation induced superabsorbent hydrogels. Acrylamide/itaconic acid copolymers. Macromol. Mater. Eng. 2001, 286, 34−42. (20) Saggu, S.; Bajpai, S. K. Water uptake behavior of poly(methacrylamide-co-N-vinyl-2-pyrrolidone-co-itaconic acid) as pHsensitive hydrogels: Part I. J. Macromol. Sci., Part A: Pure Appl.Chem. 2006, 43, 1135−1150. (21) Zhang, R.-Y.; Zaslavski, E.; Vasilyev, G.; Boas, M.; Zussman, E. Tunable pH-responsive chitosan-poly(acrylic acid) electrospun fibers. Biomacromolecules 2018, 19, 588−595. (22) For a recent review see: Cheng, B.; Pei, B.; Wang, Z.; Hu, Q. Advances in chitosan-based superabsorbent hydrogels. RSC Adv. 2017, 7, 42036−42046. (23) Ball-Jones, N. R.; Fahnhorst, G. W.; Hoye, T. R. Poly(isoprenecarboxylates) from glucose via anhydromevalonolactone. ACS Macro Lett. 2016, 5, 1128−1131. (24) Xiong, M.; Schneiderman, D. K.; Bates, F. S.; Hillmyer, M. A.; Zhang, K. Scalable production of mechanically tunable block polymers from sugar. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 8357−8362. (25) Noble, B. B.; Coote, M. L. Effects of Ionization on Tacticity and Propagation Kinetics in Methacrylic Acid Polymerization. In Controlled Radical Polymerization: Mechanisms; Matyjaszewski, K.; Sumerlin, B. S.; Tsarevsky, N. V.; Chiefari, J., Eds.; ACS Symposium Series 1187; American Chemical Society: Washington DC, 2015; pp 51−72, DOI: 10.1021/bk-2015-1187.ch003. (26) Pinner, S. H. Polymerization rate of methacrylic acid. J. Polym. Sci. 1952, 9, 282−285. (27) Blauer, G. Rate of polymerization of methacrylic acid in alkaline solution. J. Polym. Sci. 1953, 11, 189−192. (28) Max, J. J.; Chapados, C. Infrared spectroscopy of aqueous carboxylic acids: A Comparison between different acids and their salts. J. Phys. Chem. A 2004, 108, 3324−3337. (29) Flory, P. J.; Rehner, J. Statistical mechanics of cross-linked polymer networks II. J. Chem. Phys. 1943, 11, 521−526. (30) Elliott, J. E.; Macdonald, M.; Nie, J.; Bowman, C. N. Structure and swelling of poly(acrylic acid) hydrogels: Effect of pH, ionic strength, and dilution on the crosslinked polymer structure. Polymer 2004, 45, 1503−1510. (31) Riahinezhad, M.; Kazemi, K.; McManus, N.; Penlidis, A. Effect of ionic strength on the reactivity ratios of acrylamide/acrylic acid (sodium acrylate) copolymerization. J. Appl. Polym. Sci. 2014, 131, 40959. (32) Kuhn, W.; Hargitay, B.; Katchalsky, A.; Eisenberg, H. Reversible dilation and contraction by changing the state of ionization of high-polymer acid networks. Nature 1950, 165, 514−516. (33) Kuhn et al. (ref 32) showed for a series of poly(acrylic acid)based gels a plateauing (or leveling off) rather than a falloff effect for the swelling ratios at high levels of ionization (from ca. 50% to 90% ionization). These differences may be attributed to the additional mass gained by the hydrogel in the formation of sodium carboxylate salts upon addition of a titrated amount NaOH to the acrylic acid gels. The change in mass associated with exchange of sodium for hydrogen does not appear to have been accounted for in determining the swelling ratios of those samples, which would lead to a greater falloff, more similar to our observations (Figure 3). (34) Ikegami, A. Hydration and ion binding of polyelectrolytes. J. Polym. Sci., Part A: Gen. Pap. 1964, 2, 907−921. (35) Manning, G. S. The molecular theory of polyelectrolyte solutions with applications to the electrostatic properties of polynucleotides. Q. Rev. Biophys. 1978, 11, 179−246. E
DOI: 10.1021/acssuschemeng.9b00218 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
